1. Field of the Invention
The invention relates to a vertical-axis electrical machine of umbrella design, especially a hydraulic generating set, having a stator which is mounted on a foundation or is supported in a foundation pit against the pit wall there, and an essentially disc-shaped rotor with a combined thrust and guide bearing which is mounted on a bearing support structure likewise supported on the foundation.
Such an electrical machine is known, for example, from the book by Wiedemann/Kellerberger "Konstruktion elektrischer Maschinen" ("Design of electrical machines"), Springer Verlag, Berlin-Heidelberg-New York 1967, page 169, FIG. 56.
2. Discussion of Background
Generators for driving through Kaplan turbines--less frequently for Francis turbines, sometimes also for propeller turbines--are often constructed as so-called umbrella types, in which the rotor is seated overhung--that is without an upper neck bearing--on the shaft (cf. loc cit, page 167, FIG. 54 or page 169, FIG. 56). The thrust bearing is combined with the lower neck bearing, and installed in the lower bearing bracket. The bearing bracket is supported on the foundation.
Because, despite intensive cooling (by the lubricant), the thrust and guide bearing and the central region of the bearing support structure assume a substantially higher temperature than the peripheral parts of the bearing support structure, mechanical stresses arise which must be transferred to the foundation if the bearing support arrangement is rigid.
A possibility of keeping such loadings from the foundation consists in providing the bearing bracket with sloping arms or sloping spokes. In this way, preserving the centering and with only an insubstantial reduction in the radial stiffness, elongations of the central components are converted into a relative rotation between said components and the foundation (cf. German Patent 2,495,236 or U.S. Pat. No. 4,258,280). However, the solution using sloping spokes requires an expensive steel structure with a considerably greater axial height, and is only conditionally applicable to umbrella-type generators. Another known bearing support is shown in U.S. Pat. No. 4,048,528 (Whitney) which uses a box frame to support a thrust bearing of the rotor shaft. However, although the box frame is rigid, it cannot compensate for thermal elongations, and so provides undue stresses to both the foundation and the bearing.